How to link a variable of a material to another material?
I think the easiest way to do this - is using node-groups. This is instruction:


Add a node with your parameter for roughness it will be Value node:





Select your node than press Ctrl+G. It will create a node group with output from value node:





To exit from node-group press Tab and connect node group to 
roughness socket. 





In N-pannel you can change name to any that you like:





And finally, in outer material nodes you can add same node-group from Add(Shift+A)->Group menu.





New node will have number near node-group name. It indicates how many copies of this node-group you use.